1. Field of the Invention
The present invention relates to a presensitized plate, and particularly, to a presensitized plate which is excellent in thermosoftening resistance, and especially, in fatigue strength after burning treatment so that generation of cracking can be effectively prevented. Further, the present invention relates to a presensitized plate, which is capable of strongly adhering to a recording layer and excellent in press life.
2. Description of the Related Art
An aluminum plate has been heretofore employed as a support of a lithographic printing plate. In this case, a roughening treatment is generally performed on a surface of the aluminum plate in order to provide it with adhesivity to a photosensitive layer and with water-holding property at the non-image area thereof. As for a method of a surface-roughening treatment, there are known several methods such as a mechanical roughening method such as ball graining, brush graining; an electrochemical roughening method wherein the surface of the aluminum plate is electrolytically polished by use of an electrolyte mainly composed of hydrochloric acid, nitric acid, etc.; and a chemical roughening method wherein the surface of the aluminum plate is etched by use of an acid solution or an alkali solution. However, since a roughened surface obtained by means of the electrochemical roughening method is homogeneous in pits (roughness) and excellent in printing performance as the aluminum plate is formed into a lithographic printing plate, the roughening treatment is mainly performed in recent years by means of the electrochemical roughening method or by a combination of the electrochemical roughening method and another kind of roughening method.
As for the materials suitable for use in such roughening treatment, JIS A1000 type materials represented by a JIS A1050 material are frequently employed. The reason for this is that since the A1000 type materials are high in purity of aluminum and hence negligible in impurities, an electrochemical roughening treatment (electrolytic roughening treatment) or a chemical roughening treatment can be stably performed. Additionally, the A1000 type materials are also suitably applicable to the mechanical roughening treatment because of a proper mechanical strength.
Following the roughening treatment, the aluminum plate is generally subjected to an anodizing treatment in order to improve a hardness of the surface thereof and to improve the adhesion between the aluminum plate and a photosensitive layer. Subsequently, the photosensitive layer is formed on the aluminum plate to thereby obtain a presensitized plate. As required, an interface treatment or an undercoating is also generally performed prior to the formation of the photosensitive layer. The presensitized plate thus obtained is then subjected to an exposure and a development of an image, and additionally subjected to gumming if required, thereby forming a lithographic printing plate, which is then attached to a plate cylinder of a printing machine to perform printing.
When the lithographic printing plate is attached to the plate cylinder of the printing machine, both end portions of the lithographic printing plate are bent, and these bent end portions are respectively fixed to two clamps, i.e. so-called a holding portion and a holding tail portion of the plate cylinder of the printing machine in such a manner that tension is applied to the lithographic printing plate so as to enable the lithographic printing plate to be closely adhered to the plate cylinder. In the case of offset printing, when ink and fountain solution are fed to the lithographic printing plate fixed to the plate cylinder, the ink is adhered to an image area exhibiting lipophilicity while the fountain solution is adhered to a non-image area exhibiting hydrophilicity, thereby permitting the ink to be selectively disposed in correspondence with the image. The ink corresponding with the image is once transferred to an intermediate cylinder called a blanket cylinder, and then, re-transferred to paper, etc., thus obtaining a printed matter. In this case, the two bent portions formed at both ends of the lithographic printing plate are likely to be relieved up from the plate cylinder due to a reaction force to the bending of the lithographic printing plate. Accordingly, there is a problem if the plate cylinder is repeatedly pressed against the blanket cylinder under such a condition, the aforementioned relieved portion is bent repeatedly, thus inviting generation of fatigue fracture of the lithographic printing plate.
Meanwhile, the lithographic printing plate is generally subjected to a heat treatment called a burning treatment (post baking treatment) after the exposure and the development. Such burning treatment is generally performed at a temperature of 200° C. or more, in particularly at a temperature ranging from 240° C. to 270° C. though the specific temperature thereof differs depending on a purpose thereof. The photosensitive layer of the image area is further hardened by the burning treatment, thereby making it possible to improve the press life and hence to increase the number of sheets, which is attributed to the fact that since the photosensitive layer of the image area is hardened, abrasion of the photosensitive layer during the printing is suppressed.
However, in some cases, there occurs a problem that the recrystallization and restoration in the aluminum plate are caused to occur by the burning treatment, thereby lowering the mechanical strength of the aluminum plate.
There are a number of suggestions with respect to the lowering of mechanical strength after the burning treatment. For example, JP 04-73394 B and JP 07-126820 A suggest defining 0.2% proof stress or the like after the heat treatment. Further, JP 07-39906 A suggests defining a circle-corresponding diameter of a crystal grain in a cross-section of a plate. Moreover, JP 07-305133 A suggests defining the quantity of the solid solution of Fe.
There are a number of suggestions of countermeasures with respect to components of an alloy. For example, a method of adding Mn is suggested in JP 05-501585 A; U.S. Pat. No. 5,009,722 B; JP 04-19290 B; and U.S. Pat. No. 5,114,825 B. Further, a method of adding Mg is suggested in: JP 05-00462 B; JP 06-37116 B; JP 04-73392 B; JP 03-68939 B; and JP 03-11635 B. Further, a method of adding both Mn and Mg is suggested in JP 05-76530 B and JP 05-28197 B. Further, a method of adding Zr singly or in combination with Mn or Mg is suggested in JP 1992-72720 B.
According to the method of defining the 0.2% proof stress after the heat treatment as suggested by JP 04-73394 B and JP 07-126820 A, according to the method of defining the circle-corresponding diameter of a crystal grain in a cross-section of a plate as suggested by JP 07-39906 A, or according to the method of defining the quantity of the solid solution of Fe as suggested by JP 07-305133 A, it is certainly possible to minimize a lowering rate of the tensile strength after the burning treatment, and these methods are effective to some extent. However, these methods are accompanied with a problem that as a result of the repetition of printing of a large number of sheets, the fatigue fracture of the lithographic printing plate can be caused to occur.
On the other hand, although the methods of adding Mn or Mg are effective in preventing the fracture of the lithographic printing plate during the printing, but are accompanied with problems that the methods are inferior in terms of the efficiency and the stability of the roughening treatment as compared with the JIS A1000 type materials, which are excellent in applicability to the roughening treatment, and also invite increase in cost for the raw materials as the methods require predetermined trace elements as raw materials.